Magnum serpentine belt

If you have the original belt, you can cut it in one spot and use it to measure for the correct belt.
Have someone hold the tensioner just past half way (there is usually an alignment mark on the tensioner), then route the belt, clamp it at the overlap with vise grips, then release the tensioner. If the tensioner mark is lined up, mark the belt and you have the length.
The metric belt numbers start with rib count and then letter K. Next comes the three or four numbers that are the length in millimeters.